Priority is claimed to Patent Application Number 2001-5065 filed in Rep. of Korea on Feb. 2, 2001, herein incorporated by reference.
1. Field of the Invention
The present invention relates to a surface-emitting laser diode formed of a GaN series III-V nitride compound and a method for manufacturing the same, and more particularly, to a GaN series surface-emitting laser diode having a spacer for effective diffusion of holes between a p-type electrode and an active layer, and a method for manufacturing the same.
2. Description of the Related Art
As shown in FIG. 1, a general GaN series surface-emitting laser diode includes an active layer 11 of an InGaN multi-quantum well (MQW) structure, a cavity 10 having an n-AlGaN carrier barrier layer 12 under the active layer 11 and a p-AlGaN carrier barrier layer 13 on the active layer 11, each of which confines carriers to the MQW structure, and distributed Bragg reflectors (DBRs) 20 and 30 which are formed on and underneath the cavity 10, respectively, with a reflectivity of about 99%.
DBRs are classified according to materials used for the DBRs: those formed of semiconductor materials having a similar lattice constant by epitaxial growth, and those formed of dielectric materials. The former has advantages in that current can be injected through semiconductor layers and the resultant material layers have good quality. In this case, suitable semiconductor materials should have bandgap energies greater than a desired oscillation wavelength so as not to cause absorption. A greater difference in refractive index between semiconductor materials for the two DBRs is preferable. For a GaN surface-emitting laser diode, as shown in FIG. 1, suitable semiconductor materials for the DBRs 20 and 30 include GaN (for layers indicated by reference numerals 22 and 32), AlN (for layers indicated by reference numerals 21 and 31), and AlGaN. Here, AlN and AlGaN including 30% or greater Al have too large bandgap energies. For this reason, when current is injected through DBRs formed of the materials, drive voltage becomes high, causing a heat related problem. In particular, AlGaN series materials have a small difference in refractive index, and thus multiple layers, e.g., tens of layer pairs, should be deposited for DBRs to satisfy a high-reflectivity requirement for laser oscillation. Due to narrow width of a high-reflectivity region, there is a difficulty in designing surface-emitting semiconductor laser diodes. In addition, laser oscillation requirements cannot be satisfied by slight deviations in thickness of the cavity 10 or slight changes in composition of the active layer 11.
For these reasons, dielectric materials, instead of semiconductor compounds, have been widely used. In this case, current cannot be directly injected through DBRs, so a separate electrode (not shown) is required around the DBRs. The mobility of electrons is high and a doping concentration in an n-type compound semiconductor layer between an n-type electrode and an active layer can be increased. Meanwhile, the mobility of holes is smaller than that of electrons and it is impossible to increase a doping concentration in a p-type compound semiconductor layer between a p-type electrode and the active layer. Thus, there is a problem in injecting current. In addition, due to an electrode being formed around a laser output window, it is not easy to effectively diffuse holes toward the center of the laser output window and thus it is difficult to provide effective laser oscillation characteristics.
To solve the above-described problems, it is a first object of the present invention to provide a GaN series surface-emitting laser diode in which a stable optical mode is ensured by effectively diffusing holes toward the center of a laser output window.
It is a second object of the present invention to provide a method for manufacturing a GaN series surface-emitting laser diode.
To achieve the first object of the present invention, there is provided a surface-emitting laser diode comprising: an active layer; p-type and n-type material layers on opposite sides of the active layer; a first distributed Bragg reflector (DBR) layer formed on the n-type material layer; an n-type electrode connected to the active layer through the n-type material layer such that voltage is applied to the active layer for lasing; a spacer formed on the p-type material layer with a laser output window in a portion aligned with the first DBR layer, the spacer being thick enough to enable holes to effectively migrate to a center portion of the active layer; a second BDR layer formed on the laser output window; and a p-type electrode connected to the active layer through the p-type material layer such that voltage is applied to the active layer for lasing. It is preferable that the laser output window is formed in a lens-like shape having a predetermined curvature to compensate for a drop in characteristics of a laser beam caused by the spacer. It is preferable that the spacer has a protrusion portion, and the laser output window is formed on the top of the protrusion portion. It is preferable that the p-type electrode is formed to surround the protrusion portion of the spacer. It is preferable that the spacer comprises: a first spacer formed on the p-type material layer; and a second spacer formed on the first spacer on which the laser output window is formed and around which the p-type electrode is formed. It is preferable that the second spacer has a protruded shape on which the laser output window is formed. It is preferable that one of the first and second spacers is a p-type doped substrate or an undoped substrate.
To achieve the second object of the present invention, there is provided a method for manufacturing a surface-emitting laser diode, the method comprising the steps of: (a) sequentially forming a p-type material layer for lasing, an active layer, and an n-type material layer for lasing on a substrate; (b) forming a first distributed Bragg reflector (DBR) on the n-type material layer, around which an n-type electrode is formed; (c) forming a laser output window on a bottom surface of the substrate, the laser output window having a shape suitable for compensating for a drop in characteristics of a laser beam caused by the presence of the substrate; (d) forming a p-type electrode on the bottom surface of the substrate to surround the laser output window; and (e) forming a second DBR layer on the laser output window.
Preferably, step (b) comprises: forming a conductive layer on the n-type material layer; forming a mask pattern on the conductive layer to expose a portion of the conductive layer in which the first DBR layer is to be formed; removing the portion of the conductive layer which is exposed through the mask pattern, using the mask pattern as an etch mask; forming the first DBR layer on a portion of the n-type material layer from which the conductive layer is removed; and removing the mask pattern.
Preferably, in step (c), the laser output window is formed in a convex lens-like shape having a predetermined curvature suitable for compensating for diffraction of the laser beam. Preferably, in processing the mask pattern, the mask pattern is processed into a convex lens-like shape by reflowing, the convex lens-like shape having a predetermined curvature suitable for compensating for diffraction of the laser beam.
It is preferable that the substrate is formed of multiple layers including a first substrate and a second substrate on the first substrate. In this case, etching the bottom surface of the substrate on which the processed mask pattern is formed is continued until the second substrate is exposed.
It is preferable that the substrate is a p-type doped substrate or an undoped substrate. It is preferable that one of the first and second substrates is a p-type doped substrate or an undoped substrate. It is preferable that the first substrate is formed as a substrate on which a gallium nitride based material is grown and the second substrate is formed as a p-type spacer.
The present invention also provides a method for manufacturing a surface-emitting laser diode, the method comprising the steps of: (a) sequentially forming on a substrate an n-type material layer for lasing, an active layer, a p-type material layer for lasing, and a p-type spacer; (b) forming a laser output window in a predetermined area of the p-type spacer; (c) forming a p-type electrode on the p-type spacer to surround the laser output window; (d) forming a first distributed Bragg reflector (DBR) layer on the laser output window; (e) removing the substrate; and (f) forming a second DBR layer on a predetermined portion of a bottom surface of the n-type material layer and forming an n-type electrode around the second DBR layer. It is preferable that the substrate is formed of an n-type substrate or a sapphire substrate. It is preferable that the laser output window is formed in a convex lens-like shape having a predetermined curvature suitable for compensating for diffraction of the laser beam.
The surface-emitting layer diode according to the present invention comprises a spacer between a p-type electrode and an active layer to effectively cause holes to migrate towards the active layer. In addition, a DBR layer is formed on a portion of the spacer in a shape suitable for compensating for diffraction caused by the spacer and for minimizing the radius of laser mode in the active layer. Thus, with the surface-emitting laser diode according to the present invention, holes as well as electrons can effectively be provided to the center of the active layer, and thus the current threshold for laser emission is reduced. Energy conversion efficiency becomes high, and the laser beam emitted by the surface-emitting laser diode has stable transverse mode characteristics.